I feel the best way to gain muscle mass and definition is to lift weights. This is only my personal opinion based on my own experiences – sorry, no cites.
This May I embarked on my personal weight training program, and so far it has been most rewarding. I have put on somewhere between 12 and 14 pounds, most of it muscle. My muscle tone has visibly improved. Not only can I see numerically my improvement (in terms of being able to bench press 35 more pounds than when I started, and so on through all the exercises I do), I also feel much more confident when doing physical things. It’s somewhat hard to explain – when confronted with a physical challenge I just feel more capable of handling it.
In terms of diet, I just follow the food pyramid proportionally, but basically eat whatever I want. This may not work as well for you as it does for me: 1) I am 17, 2) I ride my bike to and from work, ride a stationary bike at home, and run cross-country, and 3) my job is an active one where I am outdoors for 8 or 9 hours a day. In short, I burn some serious calories.
The most important thing to do is to stick with your routine. Read some books, maybe get some professional advice or talk to friends who are weight lifters/body builders, and develop a routine based on what you’ve learned. Then, stick to it. Religiously. Never, ever, ever go 3 days without lifting. Once you are into it you may be able to afford to miss some days or restructure, but in the beginning, the most important thing is to get in the habit of lifting. You will be sore. You will hurt. You will get over it. After a few weeks you will stop getting sore from your normal regimen, and only when you increase weight will you maybe feel a little pain the next day. It won’t be like the first few weeks though. Some people write contracts with themselves, some people reward themselves after lifting, some people just grit their teeth and do it. Whatever works for you. If you absolutely cannot lift one day, then lift the next day, and if you were on a Monday-Wednesday-Friday schedule before, switch to a Tuesday-Thursday-Saturday schedule.
I personally lift free weights in my basement. This allows me to focus on what I’m doing in relative solitude, not compare myself to others at the gym, not have to wait in line for any exercises, and to listen to whatever music I want to listen to. There is only one disadvantage, but it is a major one: safety. When you are by yourself doing the bench press, if you lose control of the weight, you are in trouble. For me this has never been a problem, but you have to be extremely careful to know your own limits, and be able to recognize your body’s signs of fatigue. You’ll have to weigh (haha) the risks vs. rewards of being by yourself. If you have never lifted before in your life, you might at least want a friend with you the first few sessions, just for safety’s sake.
I would recommend free weights over any machine you can buy (I recognize this is highly debatable). Machines, while potentially safer, offer you only a certain number of exercises. With free weights, you can do nearly anything. Machines do force you into good form, but with diligence and care, you can use good form with free weights too. If you want to use a machine, I would recommend just going to a health club/school gym – they will have more machines than you can shake a stick at, and free weights to boot.
That was somewhat lengthy and IMHO-ish. It is all based on my personal experience and opinions, and while I have read several books about weight training and bodybuilding, this advice in the end is worth about what you paid for it – but it has worked well for me.